A hysteresis comparator circuit is commonly used in an output circuit section of an infrared remote control receiver or other component. The hysteresis comparator serves to prevent malfunctions of a circuit, such as chattering and the like.
In recent years, there has been a strong demand for energy savings. As to an infrared remote control receiver, there has been a need for reduction in power consumption during standby (during receiving no signal input). However, with a low bias current, it is difficult to drive a load. Thus, it is demanded to improve drive performance in an output circuit with a low current.
Further, in an IC used in an infrared remote control receiver or other component, which is provided in the vicinity of a photoelectric transfer element, light leak current occurs in a parastic photo diode (hereinafter referred to as PD) due to noise light, becoming a cause of malfunctions. Such noise light includes diffracted light and scattering light of signal light, fluorescent light, incandescent light, and the like. This will be a problem specifically for a low current circuit, since effects of such leak current are significant in a low current circuit.
Further, there has also been a demand for a low cost, and such a circuit has been demanded that has a simple structure and a less chip size.
In the following, description is made as to a comparator circuit which maintains drive current capabilities in an output circuit even under the use of a low operation current, and which is small in size and less affected by the light leak current.
FIG. 14 illustrates a block diagram illustrating an infrared remote control receiver. The infrared remote control receiver includes an fo trimming circuit 101, a detector circuit 102, an integrator circuit 103, a hysteresis comparator 104 and the like. Generally, a photocurrent signal (input current signal) I_in, inputted from a photodiode (PD) chip, is demodulated and outputted by an integrated receiving chip. The output is connected to a microcomputer or the like, which controls an electronic device. The photocurrent signal I_in is an ASK signal modulated by a specific carrier wave approximately ranging from 30 kHz to 60 kHz. In the receiving chip, the received photocurrent signal is amplified by an amplifier, and carrier elements are extracted by a band pass filter (BPF) adjusted to a frequency of the carrier wave. Then, the carrier wave is detected by a detector circuit. Further, an integrator circuit integrates a time period during which the carrier wave is present and a hysteresis comparator determines the presence or absence of the carrier wave, so that digital output is carried out. FIG. 15 represents waveforms of the sections.
As a conventional example 1, FIG. 16 illustrates a circuitry of a hysteresis comparator described in a publicly known reference 1 (Japanese Utility Model Application No. 132127/1989 (Jitsukaihei 1-132127; published on: Sep. 7, 1989). The hysteresis comparator circuit includes a hysteresis voltage generating circuit 111, a current-to-voltage conversion resistor 112 (R1), an output stage circuit 113, and a comparator circuit 114. An input stage of the comparator circuit section includes differential pairs, in each of which an NPNTr and a PNPTr (QN1 and QP1, QN2 and QP2) have a darlington connection (hereinafter, Tr refers to a transistor). With the circuitry of the conventional example 1, such a hysteresis comparator circuit is provided that realizes high input impedance and prevents malfunctions of an output circuit.
FIG. 17 is a block diagram illustrating a comparator circuit of the conventional example 1.
The following describes operation of a conventional hysteresis comparator circuit.
(i) When Vin<Vth_H during standby, the differential pair QN2 and QP2 operate. Thus, an output current Iout1 of the comparator circuit 114 becomes 0, and the output stage circuit 113 does not operate. As a result, an output current Iout of the output stage circuit 113 is expressed by Iout=0. Thus, Vo=Hi.
In the hysteresis voltage generating circuit 111, the following equation (1) is satisfied.Vth—H=Vbe(D1)+I2·R3+I1·(R2+R3)  [1]In this case, a current draw during standby is I1.
(ii) When Vin>Vth_H during receiving signal input (base currents of the transistors are ignored for simplicity), since the differential pair QN1 and QP1 operate, the following relationship is found.Iout1=I1In the output stage circuit 113, QP5 turns on when R1·I1>Vbe. This allows the following relationship to be found, and finally gives the relationship expressed by equation (2).Iout2=Is·exp(R1·I1/Vt),Iout=m·Is·exp(R1·I1/Vt)  (2),where Is is a saturation current of a transistor, Vt=kT/q, k is Voltzmann constant, Q is an elementary electric charge of an electron, T is an absolute temperature, m is a current ratio between a current mirrored current of QN5 and a current mirrored current of QN6. Thus, Vo=Lo.
In the hysteresis voltage generating circuit 111, the following equation (3) is satisfied.Vth—L=Vbe(D1)+I2·R3  (3)The hysteresis voltage is expressed by I1·(R2+R3), and the drive current is determined by the equation (2).
In the conventional example 1, the comparator circuit 114 has a current draw during standby expressed by I1, and a drive current expressed by the equation (2), allowing for trade-off between reduction in the current draw and drive current. That is, reduction in current draw causes reduction in drive current.
According to the equation (2), the drive current is increased by increasing m. However, since transistor(s) in an output stage are generally large, a chip size is increased, causing a cost increase.
Further, according to the equation (2), the drive current is increased by increasing R1. The following problems occur in this case.
In an IC provided in the vicinity of a photoelectric transfer element (e.g. an IC used in a remote control receiver), light leak current occurs in a parastic photo diode due to noise light, becoming a cause of malfunctions in many cases. Such noise light includes diffracted light and scattering light of signal light, fluorescent light, incandescent light, and the like. FIG. 18 illustrates a structure of a L (lateral, horizontal type) PNPTr, and FIG. 19 illustrates an equivalent circuit of the LPNPTr. BS, EM, and CL denote a base, an emitter, and a collector, respectively. S1 is an area of an epitaxial layer of N type, and S2 is an area of the emitter. Due to structural features of an integrated circuit, a parastic PD, i.e., PDa, exists between the epitaxial layer of N type and a substrate of P type. Thus, a parastic PD is connected between a base terminal of the PNPTr and ground. Due to leak current occurred in the parastic PD, a base current of the LPNPTr is increased, giving significant effects of characteristics of the circuit.
FIG. 20 illustrates a structure of an NPNTr, and FIG. 21 illustrates an equivalent circuit of the NPNTr. Similarly, a parastic PD exists between an epitaxial layer of N type and a substrate of P type. Thus, a parastic PD is connected between a collector terminal of the NPNTr and ground. This causes an increase in collector current of the NPNTr, giving significant effects of characteristics of the circuit.
In the conventional example 1, a light leak current occurred due to a parastic PD is amplified to hfe(pnp) times by QP1. The parastic PD varies depending on (i) collector diffusion of an NPNTr and (ii) base diffusion of a PNPTr, each of which is provided in an input section of a differential amplifier in a hysteresis comparator circuit. This allows the following relationship to be found.Ileak=hfe(pnp)·{Ipd(npn)+Ipd(pnp)}  (4),where hfe(pnp) is a current amplification factor of a PNPTr, Ipd(npn) is a light leak current occurred due to collector diffusion of an NPNTr, and Ipd(pnp) is a light leak current occurred due to base diffusion of a PNPTr. Ileak causes a voltage drop in R1. When a relationship expressed by the following equation (5) is satisfied, QP5 turns on and the circuit malfunctions.R1·hfe(pnp)·{Ipd(npn)+Ipd(pnp)}>Vbe(QP5)  (5)
As clearly seen in the equation (5), increasing R1 increases the effects of the light leak current.